The invention is based on a priority application EP 03027017.7 which is hereby incorporated by reference.
This invention relates to the field of telecommunication and, more particularly, to spread spectrum telecommunication systems.
In a typical RF communication system, a transmitted signal may travel from a transmitter to a receiver over multiple paths, for example a direct path and also a reflected path. Each path may be considered a separate channel which is subject to the effects of fading, dispersion, etc. Moreover, the combination of signals at the receiver can result in additional fading. Such operating environments are known as multipath fading environments. Direct sequence spread spectrum (DS-SS) receivers can operate in multipath fading environments. A DS-SS receiver typically includes a Rake receiver, which demodulates a received signal using plural demodulation “fingers”, often referred to as Rake fingers. Each Rake finger demodulates the component signal from a number of the channel paths (such component signal referred to as a multipath component). The outputs of the Rake fingers are combined for improved performance.
With multipath channels a transmitted signal arrives in components, with each component having a different delay. The components can be distinguished and resolved if the delays are of sufficient duration. However, in order to demodulate the signals, the Rake receiver must know the delay of each channel path.
Typically, a Rake receiver operates in conjunction with a delay searcher and a delay tracker. The delay searcher analyzes a received signal and finds the delays. These delays are assigned to the Rake fingers. However, in mobile telecommunications the channels may be subject to additional fading due to the motion of the receiver. A delay tracker tracks the delays assigned by the searcher between channel searches. Thus, while the searcher looks over a wide range of delays, the trackers look over a smaller range surrounding the assigned delays.
U.S. Pat. No. 6,560,273 shows a DS-SS receiver for operating in a multipath fading channel. The system comprises a Rake receiver having plural Rake fingers. Each Rake finger demodulates a received signal from one of a plurality of channel paths. The output of the plural Rake fingers are combined. Each Rake finger utilizes a select assigned delay to synchronize to a delay of the one channel path. A searcher periodically performs a channel search on the received signal to detect new delays of strongest paths in the channel. Plural trackers, one for each channel path, adjust the select assigned delays between searches performed by the searcher. A delay controller is operatively coupled to the searcher and the trackers. The delay controller compares new delays of the strongest paths from the searcher to the select assigned delays and reassigns one of the select assigned delays with one of the new delays only if the new delay differs from the one select assigned delay more than a predetermined threshold.
U.S. Pat. No. 6,430,166 is aimed to increase the performance of a demodulator in a CDMA receiver by cascading of path searchers in order to obtain an optimized channel profile.
U.S. Pat. No. 6,370,183 shows a Rake receiver with a finger for each of a plurality of multipath components displaced in time. Short-term variations in the strength of each component are predicted and the coefficients of each finger are adapted accordingly, with the aim to reduce the error rate, particularly in high-data-rate applications in a time-variant environment.
Further, delay spread and its impact on Rake reception in spread spectrum telecommunication systems is discussed in the scientific literature (cf. Impact of spreading bandwidth on RAKE reception in dense multipath channels Communication Theory Mini-Conference, 1999, Win, M. Z.; Kostic, Z. Pages: 78-82; Performance of RAKE reception in dense multipath channels: implications of spreading bandwidth and selection diversity order. Selected Areas in Communications, IEEE Journal on, Win, M. Z.; Chrisikos, G.; Sollenberger, N. R. Pages: 1516-1525; Impact of spreading bandwidth on RAKE reception in dense multipath channels Selected Areas in Communications, IEEE Journal on, Win, M. Z.; Kostic, Z. A. Pages: 1794-1806; Virtual path analysis of selective RAKE receiver in dense multipath channels Communications Letters, IEEE, Win, M. Z.; Kostic, Z. A. Pages: 308-310).
The present invention aims to provide an improved method of finding delays of a multipath channel, in particular for the purpose of Rake reception of spread spectrum signals.